In recent years, a light emission efficiency of a light-emitting diode device has been extremely improved, and an application of the light-emitting diode device to an illumination is being progressed. In particular, in the case in which there is used a light emitting diode device as a back light for a liquid crystal display, a satisfactory gamut of reproducible colors and a high speed response can be implemented and it is expected that a high quality display be achieved (Japanese Patent Publication No. 3576541).
Conventionally, the main stream of such a back light for a liquid crystal display has been the so-called edge light type in which a cold cathode tube as a light source is disposed on the edge face of the chassis for thinning and low power consumption of the apparatus (Japanese Patent Publication No. 2732492). However, a demand of enlarging a liquid crystal display has been increased in recent years, and the edge light type has a limitation in improving a luminance and a uniformity in the luminance. Therefore, an adoption of a direct lighting type light has been examined for a large size liquid crystal display.
FIG. 5 is a cross sectional view for showing a configuration of a conventional planar light source device of a direct lighting type used in a liquid crystal display (TECHNO-FRONTIER SYMPOSIUM 2005, Thermal Design and Countermeasure Technology Symposium, Issue Date: Apr. 20, 2005 (Japan Management Association), Session G3: Latest Design Case of Heat Radiation Mounting I (pp. G3-3-1 to G3-3-4) and so on). The flat light source device 11 is disposed directly below a liquid crystal panel 21. In the flat light source device 11, LED light sources 1 using a light emitting diode device are disposed in an array pattern on a bottom face of a chassis 12, and the LED light sources 1 have a plurality of luminescent colors different from each other such as three colors of RGB. Moreover, the bottom face and side faces of the chassis 12 are covered with a reflecting sheet 13. Furthermore, over the LED light sources 1, a diffusing sheet 14 and a prism sheet 15 are disposed in the range of 1 to 5 cm in general apart from the LED light sources 1.
In the case in which a light is emitted from the LED light sources 1, the emitted light travels directly toward the diffusing sheet 14, or is reflected by the reflecting sheet 13 and travels toward the diffusing sheet 14. The emitted light is then irregularly reflected in the diffusing sheet 14, and is inclined in a vertical direction by passing through the prism sheet 15. The emitted light then enters a liquid crystal panel 21. Lights emitted from different LED light sources 1 are mixed in a space between the LED light sources and the diffusing sheet 14. The mixing is then improved by an irregular reflection in the diffusing sheet 14, thereby implementing a uniform luminance and a uniform chromaticity.
However, in the case in which a color mixture is carried out by using LED light sources of a plurality of colors such as RGB without using a monochromatic LED light source, a color mixture is insufficient and unevenness of colors may be found.
Moreover, in the case in which the diffusing sheet is made further apart from the LED light sources, unevenness of colors can be reduced to a certain level by a spatial color mixture. However, this method causes a thickness of a back light to be enlarged, and is not preferable for a flat panel display.
In the above described flat light source device of a direct lighting type, in order to further improve a uniform chromaticity and suppress a thickness of the flat light source device, it is effective to dispose a light guide plate that faces to LED light sources in front of a substrate in which a plurality of LED light sources is disposed and to dispose a reflecting layer on the back face of the light guide plate, thereby propagating and diffusing a light that has been emitted from each LED light source in a horizontal direction in the light guide plate and then extracting the light forward.
However, even in the case in which such a light guide plate is used, the disposed position of the LED light sources has a certain restriction in consideration of a heat radiation from an LED chip and a circuit design of a substrate on which an LED chip is disposed. Consequently, the relative positions of luminous origins of luminance colors different from each other such as RGB are restricted, thereby causing a sufficient color mixture to be restricted.
As described above, it is necessary to use LED light sources with a plurality of colors such as RGB in order to obtain a white flat light source such as a back light for a liquid crystal display. On the other hand, a means of obtaining a white color without a color mixture by LED light sources with a plurality of colors is thought to be using a white LED light source (Japanese Laid-Open Patent Publication No. 2004-55772 and Japanese Laid-Open Patent Publication No. 2004-70193). For a known conventional white LED light source, a resin layer in which the inorganic fluorescent material particles such as YAG-Ce have been dispersed is formed on the periphery of an LED bare chip that has been face-down bonded to a substrate, and, for instance, a blue light emitted from a blue LED bare chip is mixed to a yellow fluorescence emitted from the inorganic fluorescent material particles that has been excited by a light emitted from the blue LED bare chip, thereby emitting a white color light as a whole.
However, although a light emitted from the above white LED light source is visually white, a luminance characteristic in a visible light wave length area is not flat and a wavelength distribution of a luminance has a deflection. For a liquid crystal display, a color filter of RGB is disposed on a liquid crystal panel, and each color of RGB is extracted for every picture element through a color filter which a white light emitted from a white back light passes. However, for the above white LED light source, since a blue LED light and a yellow fluorescence is mixed to obtain a white color, a strength of a red color area that is on the longest wave side of a visible light wave length area is weaker than a strength of other wave length areas. Consequently, a color of a light that has passed through a red filter is unnatural and a color rendering property is not satisfactory.
As a fluorescent material of a conventional white LED light source, an inorganic particle is used as described above. However, in the case in which a resin plate in which such inorganic particles are dispersed is used for a light guide plate to which a sufficient light propagation property is required, the inorganic particle becomes a dispersion origin and prevents a light propagation in a horizontal direction, and the function of a light guide plate for diffusing lights emitted from each LED light source is lost.
The above light source in which an LED bare chip is combined with a fluorescent material of a single kind is not indicated to be applied to a light guide plate having functions for introducing lights emitted from each LED light source disposed on the back side into one plate having a light propagation property, for propagating and diffusing the lights in a horizontal direction in the plate, and for emitting the lights forward.
An object of the present invention is to provide a light guide member, a flat light source device using the light guide member, and a display device, by which a white light having a high color rendering property can be obtained without using luminescent devices of a plurality of colors.